added check if enemy exists before accesing droptable to prevent game crash

scripts/drop_manager.gd

@@ -15,6 +15,8 @@ func on_enemy_died(enemy):
 	call_deferred("drop_item", enemy)
 	
 func drop_item(enemy):
+	if not is_instance_valid(enemy):
+		return
 	for entry in enemy.drop_table:
 		if randf() < entry.chance:
 			var drop = entry.drop.instantiate()